AC MACHINES-1 (66761) Theory

Describe the direct Online starter method, Start-delta starter method (manual and

automatic), Auto transformer starter method of starting squirrel cage induction motor.

Direct On Line Starter

Direct On Line Starter Method is a common method of starting of Cage Induction Motor. The motor is connected through a starter across the full supply voltage. The Direct On Line Starter Method figure is shown below. It consists a coil operated contactor C controlled by start and stop push button as shown in the connection diagram below.

The buttons which may be installed in a convenient place away from the starter. The start button is held open by a spring. On pressing the START pushbutton S₁, the contactor C is energised from two line conductors L₁ and L₂.

Theory of Direct On Line Starting of Induction Motor

Let,

• I_st be the starting current drawn from the supply mains per phase.
• I_fl is the full load current drawn from the supply mains per phase.
• Ʈ_est is the starting torque.
• S_fl is the slip at full load.      

  Star Delta Starter

  The Star Delta Starter is a very common type of starter and is used extensively as compared to the other type of starting methods of the induction motor. A star delta is used for a cage motor designed to run normally on the delta connected stator winding. The connection of a three-phase induction motor with a star delta starter is shown in the figure below.

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When the switch S is in the START position, the stator windings are connected in the star as shown below.

When the motor picks up the speed, about 80 percent of its rated speed, the switch S is immediately put into the RUN position. As a result, a stator winding which was in star connection is changed into DELTA connection now. The delta connection of the stator winding in shown in the figure below.

Firstly, the stator winding is connected in star and then in Delta so that the starting line current of the motor is reduced to one-third as compared to the starting current with the windings connected in delta. At the starting of an induction motor when the windings of the stator are star connected, each stator phase gets a voltage V_L/√3. Here V_L is the line voltage.

Theory of Star Delta Starter Method of Starting of Induction Motor

At the starting of the induction motor, stator windings are connected in star and, therefore, the voltage across each phase winding is equal to 1/√3 times the line voltage.

Let,

• V_L is the line voltage
• I_styp is the starting current per phase with the stator windings connected in star.
• I_styl is the starting line current with the stator winding in the star

For star connection, the line current is equal to the phase current

Therefore,

Since the developed torque is proportional to the square of the voltage applied to an induction motor. Star delta starter reduces the starting torque to one-third that is obtained by direct delta starting.

Auto transformer Starter

An Auto transformer Starter is suitable for both star and delta connected motors. In this method, the starting current is limited by using a three-phase auto transformer to reduce the initial stator applied voltage. The figure below shows the motor with the Auto transformer starter.

It is provided with a number of tappings. The starter is connected to one particular tapping to obtain the most suitable starting voltage. A double throw switch S is used to connect the auto transformer in the circuit for starting. When the handle H of the switch S in the START position. The primary of the auto transformer is connected to the supply line, and the motor is connected to the secondary of the auto transformer.

When the motor picks up the speed of about 80 percent of its rated value, the handle H is quickly moved to the RUN position. Thus, the auto transformer is disconnected from the circuit, and the motor is directly connected to the line and achieve its full rated voltage. The handle is held in the RUN position by the under voltage relay.

If the supply voltage fails or falls below a certain value, the handle is released and returns to the OFF position. Thermal overload relays provide the overload protection.

Theory of Auto transformer Starter

The figure (a) shown below shows the condition when the motor is directly switched on to lines and the figure (b) shows when the motor is started with the help of auto transformer.

Let,

• Z_e10 is the equivalent standstill impedance per phase of the motor referred to the stator side
• V₁ is the supply voltage per phase.

When the full voltage V₁ per phase is applied to the direct switching, the starting current drawn from the supply is given by the equation shown below

 Entire stator connection for a star-delta starter.

Wiring diagram (left) and line diagram of star-delta starter (right).

Elementary diagram of sizes 1, 2, 3, 4, 5, and 6 star-delta starters with transition starting.